1. Field of the Invention
The present invention relates to a hydraulic fluid pressure control system, included in an automatic transmission, for controlling the shifting operation of a gear transmission mechanism which includes a plurality of friction engaging devices, and more particularly relates to an improvement of such a hydraulic fluid pressure control system, in which a downshift timing subsystem is provided for ensuring good downshift timing from a certain speed stage of the gear transmission mechanism to the next lower speed stage.
2. Discussion of the Background
Various different types of gear transmission mechanism are used in automatic transmissions, and in these gear transmission mechanisms typically each of the speed stages is provided by supplying and exhausting actuating hydraulic fluid pressure to and from particular appropriate hydraulic pressure actuated friction engaging devices such as hydraulic clutches and hydraulic brakes, incorporated in the gear transmission mechanism. In the typical and often used gear transmission mechanisms, a certain lower speed stage is provided by a first friction engaging device being supplied with hydraulic fluid pressure to be engaged thereby and a second friction engaging device is not supplied with hydraulic fluid pressure to be disengaged thereby, and a certain higher speed stage is provided by the supply and the non supply of hydraulic fluid pressure to said first and second friction engaging devices being reversed. This type of speed stage shifting for a gear transmission mechanism is generally referred to as switchover shifting. Typically, such a hydraulic pressure control system includes a speed stage shift valve adapted to be operated by the balance between the governor hydraulic fluid pressure which represents the current road speed of the vehicle and the throttle pressure which represents the current engine load.
When a gear transmission mechanism of the switchover shifting type is shifted down from a higher speed stage to a lower speed stage, the hydraulic fluid pressure which has been supplied to said second friction engaging device is drained while hydraulic fluid pressure is newly supplied to said first friction engaging device. In this case, if the vehicle speed at the time of starting the downshifting is higher more time is required for accomplishing the downshifting without causing any undesirable shocking. Therefore, it is desirable that the supply of hydraulic fluid pressure to said first friction engaging device during the downshifting should be done more gradually as the vehicle speed at the beginning of the downshifting is higher. In order to effect such control, it has already been proposed to incorporate a downshift timing control valve in the passage for supplying and exhausting hydraulic fluid pressure to said first friction engaging device, said downshift timing control valve being adapted to respond to the governor pressure so as to apply an additional flow resistance to said passage when the governor pressure is higher than a predetermined level.
However, such downshift timing control valve incorporated in the passage for supplying and exhausting hydraulic fluid pressure to and from said first friction engaging device inevitably increases the overall minimum flow resistance of the passage, and it was found by the inventors of the present application that the increase of the minimum flow resistance of said passage caused by the provision of the downshift timing control valve delays the exhausting of hydraulic fluid pressure from said first friction engaging device during upshifting of the gear transmission beyond a limit acceptable to adjust the timing during upshifting to be optimum in addition to the adjustment of the downshift timing.